This invention relates to a copper alloy suited as material for leads of semiconductor devices such as transistors and integrated circuits (ICs) and also as material for electrically conductive springs for connectors, terminals, relays, switches and the like.
As materials for leads of semiconductor devices, high nickel alloys such as Kovar (Fe-29Ni-16Co) and 42 alloy (Fe-42Ni) have been used by preference because of their low thermal expansion coefficients and abilities to bond and seal elements and ceramics. In recent years, more and more large-power-consuming ICs have come into use with increases in the integration degree of semiconductor circuitry. Also, resins are in wider use than before as sealing material, and improvement have been made in techniques for bonding elements and lead frames. Accordingly, copper-base alloys having higher thermal conductivity are favored today over the nickel alloys as lead materials for leads.
Generally, a material for forming leads of semiconductor devices is required to have the following properties:
(1) Excellent thermal and electric conductivities, because leads must function as parts for transmitting electric signals and also function to release to the outside the heat generated during the packaging process and while the circuit is in use.
(2) A thermal expansion coefficient close to that of the mold material so as to ensure good adhesion of the leads to the mold which is important from the viewpoint of semiconductor element protection.
(3) Sufficient thermal resistance to withstand various heating steps involved in the packaging.
(4) Good machinability since most leads are made by stamping or bending the material.
(5) High plate adhesion which facilitates precious metal plating of the lead surface.
(6) Good solderability because the lead portions exposed from the sealing material after packaging, known as outer leads, are often soldered subsequently.
(7) Adequate corrosion resistance for the sake of reliability and life of the devices with the leads.
(8) Low cost.
The whole set of these property requirements have not been met by any single one of existing alloys that have merits and demerits, such as oxygen-free copper, tin copper, iron copper, phosphor bronze, Kovar, and 42 alloy.
For the fabrication of springs for electric devices and apparatuses and also for instruments, switches, connectors and so forth have been employed inexpensive brass, nickel silver having outstanding spring properties and corrosion resistance, and phosphor bronze with prominent spring properties. However, brass has poor strength and spring properties. Nickel silver and phosphor bronze do possess excellent strength and spring properties, but they are expensive alloys due partly to the material cost since they contain 18% by weight nickel and 8% tin, respectively, and partly to working limitations including poor hot machinability. They exhibit an additional disadvantage of low electric conductivities when used in electric equipment and the like. Introduction of low-cost alloys with excellent spring properties has, therefore, been eagerly waited in the art.